Logical Fallacies

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Argumentum ad antiquitatem

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This is the familiar argument that some policy, behavior, or practice is right or acceptable because "it's always been done that way." This is an extremely popular fallacy in debate rounds; for example, "Every great civilization in history has provided state subsidies for art and culture!" But that fact does not justify continuing the policy.

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Argumentum ad hominem

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(argument directed at the person). This is the error of attacking the character or motives of a person who has stated an idea, rather than the idea itself. The most obvious example of this fallacy is when one debater maligns the character of another debater (e.g, "The members of the opposition are a couple of fascists!"), but this is actually not that common. A more typical manifestation of argumentum ad hominem is attacking a source of information -- for example, responding to a quotation from Richard Nixon on the subject of free trade with China by saying, "We all know Nixon was a liar and a cheat, so why should we believe anything he says?" Argumentum ad hominem also occurs when someone's arguments are discounted merely because they stand to benefit from the policy they advocate -- such as Bill Gates arguing against antitrust, rich people arguing for lower taxes, white people arguing against affirmative action, minorities arguing for affirmative action, etc. In all of these cases, the relevant question is not who makes the argument, but whether the argument is valid.

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Argumentum ad ignorantiam

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(argument to ignorance). This is the fallacy of assuming something is true simply because it hasn't been proven false. For example, someone might argue that global warming is certainly occurring because nobody has demonstrated conclusively that it is not.

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Argumentum ad logicam

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(argument to logic). This is the fallacy of assuming that something is false simply because a proof or argument that someone has offered for it is invalid; this reasoning is fallacious because there may be another proof or argument that successfully supports the proposition.

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Argumentum ad misericordiam

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(argument or appeal to pity). The English translation pretty much says it all. Example: "Think of all the poor, starving Ethiopian children! How could we be so cruel as not to help them?" The problem with such an argument is that no amount of special pleading can make the impossible possible, the false true, the expensive costless, etc.

It is, of course, perfectly legitimate to point out the severity of a problem as part of the justification for adopting a proposed solution. The fallacy comes in when other aspects of the proposed solution (such as whether it is possible, how much it costs, who else might be harmed by adopting the policy) are ignored or responded to only with more impassioned pleas. You should not call your opposition down for committing this fallacy unless they rely on appeals to pity to the exclusion of the other necessary arguments.

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Argumentum ad nauseam

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(argument to the point of disgust; i.e., by repitition). This is the fallacy of trying to prove something by saying it again and again. But no matter how many times you repeat something, it will not become any more or less true than it was in the first place. Of course, it is not a fallacy to state the truth again and again; what is fallacious is to expect the repitition alone to substitute for real arguments.

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Argumentum ad numerum

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(argument or appeal to numbers). This fallacy is the attempt to prove something by showing how many people think that it's true. But no matter how many people believe something, that doesn't necessarily make it true or right. Example: "At least 70% of all Americans support restrictions on access to abortions." Well, maybe 70% of Americans are wrong!

This fallacy is very similar to argumentum ad populum

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Argumentum ad populum

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(argument or appeal to the public). This is the fallacy of trying to prove something by showing that the public agrees with you.

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Argumentum ad verecundiam

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(argument or appeal to authority). This fallacy occurs when someone tries to demonstrate the truth of a proposition by citing some person who agrees, even though that person may have no expertise in the given area. For instance, some people like to quote Einstein's opinions about politics (he tended to have fairly left-wing views), as though Einstein were a political philosopher rather than a physicist.

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Circulus in demonstrando

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(circular argument). Circular argumentation occurs when someone uses what they are trying to prove as part of the proof of that thing. Here is one of my favorite examples (in pared down form): "Marijuana is illegal in every state in the nation. And we all know that you shouldn't violate the law. Since smoking pot is illegal, you shouldn't smoke pot. And since you shouldn't smoke pot, it is the duty of the government to stop people from smoking it, which is why marijuana is illegal!"

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Complex question.

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question that implicitly assumes something to be true by its construction, such as "Have you stopped beating your wife?" A question like this is fallacious only if the thing presumed true (in this case, that you beat your wife) has not been established.

Complex questions are a well established and time-honored practice in debate, although they are rarely so bald-faced as the example just given. Complex questions usually appear in cross-examination or points of information when the questioner wants the questionee to inadvertently admit something that she might not admit if asked directly. For instance, one might say, "Inasmuch as the majority of black Americans live in poverty, do you really think that self-help within the black community is sufficient to address their problems?" Of course, the introductory clause about the majority of black Americans living in poverty may not be true (in fact, it is false), but an unwary debater might not think quickly enough to notice that the stowaway statement is questionable. This is a sneaky tactic, but debate is sometimes a sneaky business.

You wouldn't want to put a question like that in your master's thesis, but it might work in a debate. But be careful -- if you try to pull a fast one on someone who is alert enough to catch you, you'll look stupid. "The assumption behind your question is simply false. The majority of blacks do not live in poverty. Get your facts straight before you interrupt me again!"

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Dicto simpliciter

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(spoken simply, i.e., sweeping generalization). This is the fallacy of making a sweeping statement and expecting it to be true of every specific case -- in other words, stereotyping. Example: "Women are on average not as strong as men and less able to carry a gun. Therefore women can't pull their weight in a military unit." The problem is that the sweeping statement may be true (on average, women are indeed weaker than men), but it is not necessarily true for every member of the group in question (there are some women who are much stronger than the average).

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Appeal to Nature

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This is the fallacy of assuming that whatever is "natural" or consistent with "nature" (somehow defined) is good, or that whatever conflicts with nature is bad. For example, "Sodomy is unnatural; anal sex is not the evolutionary function of a penis or an anus. Therefore sodomy is wrong." But aside from the difficulty of defining what "natural" even means, there is no particular reason to suppose that unnatural and wrong are the same thing. After all, wearing clothes, tilling the soil, and using fire might be considered unnatural since no other animals do so, but humans do these things all the time and to great benefit.

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Naturalistic fallacy
or
The Is-Ought fallacy

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This is the fallacy of trying to derive conclusions about what is right or good (that is, about values) from statements of fact alone. This is invalid because no matter how many statements of fact you assemble, any logical inference from them will be another statement of fact, not a statement of value. If you wish to reach conclusions about values, then you must include amongst your assumptions (or axioms, or premises) a statement of value. Once you have an axiomatic statement of value, then you may use it in conjunction with statements of fact to reach value-laden conclusions.

For example, someone might argue that the premise, "This medicine will prevent you from dying" immediately leads to the conclusion, "You should take this medicine." But this reasoning is invalid, because the former statement is a statement of fact, while the latter is a statement of value. To reach the conclusion that you ought to take the medicine, you would need at least one more premise: "You ought to try to preserve your life whenever possible."

The naturalistic fallacy appears in many forms. Two examples are argumentum ad antiquitatem (saying something's right because it's always been done that way) and the appeal to nature (saying something's right because it's natural). In both of these fallacies, the speaker is trying to reach a conclusion about what we ought to do or ought to value based solely on what is the case. David Hume called this trying to bridge the "is-ought gap," which is a nice phrase to use in debate rounds where your opponent is committing the naturalistic fallacy.

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Non Sequitur

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"This does not follow" This is the simple fallacy of stating, as a conclusion, something that does not strictly follow from the premises. For example, "Racism is wrong. Therefore, we need affirmative action." Obviously, there is at least one missing step in this argument, because the wrongness of racism does not imply a need for affirmative action without some additional support (such as, "Racism is common," "Affirmative action would reduce racism," "There are no superior alternatives to affirmative action," etc.).

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Petitio principii

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(begging the question). This is the fallacy of assuming, when trying to prove something, what it is that you are trying prove. For all practical purposes, this fallacy is indistinguishable from circular argumentation.


The main thing to remember about this fallacy is that the term "begging the question" has a very specific meaning. It is common to hear debaters saying things like, "They say pornography should be legal because it is a form of free expression. But this begs the question of what free expression means." This is a misuse of terminology. Something may inspire or motivate us to ask a particular question without begging the question. A question has been begged only if the question has been asked before in the same discussion, and then a conclusion is reached on a related matter without the question having been answered. If somebody said, "The fact that we believe pornography should be legal means that it is a valid form of free expression. And since it's free expression, it shouldn't be banned," that would be begging the question.

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Post hoc ergo propter hoc

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(after this, therefore because of this). This is the fallacy of assuming that A caused B simply because A happened prior to B. A favorite example: "Most rapists read pornography when they were teenagers; obviously, pornography causes violence toward women." The conclusion is invalid, because there can be a correlation between two phenomena without one causing the other. Often, this is because both phenomena may be linked to the same cause. In the example given, it is possible that some psychological factor -- say, a frustrated sex drive -- might cause both a tendency toward sexual violence and a desire for pornographic material, in which case the pornography would not be the true cause of the violence.

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Red Herring

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This means exactly what you think it means: introducing irrelevant facts or arguments to distract from the question at hand. For example, "The opposition claims that welfare dependency leads to higher crime rates -- but how are poor people supposed to keep a roof over their heads without our help?" It is perfectly valid to ask this question as part of the broader debate, but to pose it as a response to the argument about welfare leading to crime is fallacious

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slippery slope fallacy

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says adopting one policy or taking one action will lead to a series of other policies or actions also being taken, without showing a causal connection between the advocated policy and the consequent policies. A popular example of the slippery slope fallacy is, "If we legalize marijuana, the next thing you know we'll legalize heroin, LSD, and crack cocaine."

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Tu quoque

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("you too"). This is the fallacy of defending an error in one's reasoning by pointing out that one's opponent has made the same error. An error is still an error, regardless of how many people make it. For example, "They accuse us of making unjustified assertions. But they asserted a lot of things, too!"